A single rabbit class I MHC (RLA) heavy chain has been detected in the rabbit lymphoid cell line RL-5 using conventional class I immunoprecipitation methods (anti-Beta-2-microglobulin; monoclonal antibodies). To investigate RLA structure and expression at the nucleic acid level, an Okayama/Berg cDNA library was contructed from size-selected RL-5 poly-A mRNA, and pR9, a full-length cDNA clone, encoding this RLA heavy chain, was identified. pR26, a cDNA clone distinct from pR9, encodes a class I MHC protein product not detected in RL-5 cells. To study the genomic class I locus, Lambda19-1, a genomic clone isolated from an RL-5 Lambda1 library and encoding the pR9 mRNA, is currently being sequenced. These studies employing recombinant DNA techniques will be extended to further elucidate RLA gene organization and expression.